[XRay][clang] Fix the -fxray-instruction-threshold flag processing

Summary:
The refactoring introduced a regression in the flag processing for
-fxray-instruction-threshold which causes it to not get passed properly.
This change should restore the previous behaviour.

Reviewers: rnk, pelikan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31491

llvm-svn: 299126